1. Field of the Invention
The present invention relates to a system for detecting underground objects by a synthetic aperture method.
2. Description of the Prior Art
In FIG. 5 is shown a general form of a so-called synthetic aperture underground object detecting system which is structured of a general form of the synthetic aperture processing in use for satellite-radar and airborne radar and, in addition thereto, means for determination of geological constant and geological correction using the same, which is indispensable to detection of underground objects. Examples of prior art detecting systems are disclosed in a paper entitled "Underground Detection by Means of Reflected Electromagnetic Wave (Part 2)", pages 59-60, Transactions of Japan Society of Physical Sensing, October 1982, and in a paper entitled "Electromagnetic Detection of Underground Objects", pages 308-311, Proceedings of the Institute of Electronics and Communication Engineers of Japan, Vol. 67, No. 3, March 1984. In the explanatory chart of the general form in FIG. 4, ST 1 in a step of collecting reflected wave profile data, ST2 is a step of performing preprocessing in succession to step ST1, ST3 is a step of performing synthetic aperture processing in succession to step ST2, ST4 is a step of performing geological correction in succession to step ST3, and ST5 is a step of performing output processing in succession to step ST4. And ST7 is a step of collecting geological data, ST8 is a step of analytical processing of the geological data in succession to step ST7, and ST9 is a step of calculating dielectric constant .epsilon..sub.S in succession to step ST8, and the calculated dielectric constant .epsilon..sub.S is used in the aforementioned steps ST3 and ST4 for performing synthetic aperture processing and geological correction.
FIG. 5 is an explanatory drawing showing an example of the geological data collection performed at the aforesaid step ST7. Referring to the figure, reference numeral 1 denotes a target such as a pipe, 2 denotes soil in which the target 1 is buried, 3 denotes a transmitter, 4 denotes a transmitting antenna connected with the transmitter 3 for emitting a pulse signal from the transmitter 3 as an electromagnetic wave into the soil 2, 5 denotes a receiving antenna disposed at adjustable distance from the transmitting antenna 4 for receiving the reflected wave by the target 1 of the aforesaid electromagnetic wave, and 6 denotes a receiver connected with the receiving antenna 5.
The operations will be described below. First, at step ST7, geological data are collected. That is, the distance between the transmitting antenna 4 and the receiving antenna 5 are set to Y.sub.1 and a monocyclic pulse, for example, is delivered from the transmitter 3. The monocyclic pulse is emitted as an electromagnetic wave by the transmitting antenna 4 into the soil 2 and its reflected wave is received by the receiving antenna 5 and sent to the receiver 6. Then, the distance between the transmitting antenna 4 and the receiving antenna 5 is changed to Y.sub.2 and the transmission of the monocyclic pulse and the receipt of its reflected wave are performed again. The thus obtained geological data are analytically processed at step ST8, and thereby, the period of time T.sub.1 from the transmission of the monocyclic pulse to the receipt of its reflected wave by the target 1 when the inter-antenna distance was Y.sub.1 and the period of time T.sub.2 from the transmission of the monocyclic pulse to the receipt of its reflected wave by the target 1 when the inter-antenna distance was Y.sub.2 are obtained.
Now, representing the buried depth of the target 1 by R and the actual dielectric constant of the soil 2 by .epsilon..sub.S, the following relationship holds between the period of time T from the transmission of the pulse signal to the receipt of the reflected wave by the target 1 and the inter-antenna distance Y ##EQU1## where C is the velocity of light. Therefore, substituting the aforesaid periods of time T.sub.1, T.sub.2 for T, and the inter-antenna distances Y.sub.1, Y.sub.2 for Y, and solving the simultaneous equations having .epsilon..sub.S and R as the unknown quantities, the actual dielectric constant .epsilon..sub.S can be obtained. The actual dielectric constant .epsilon..sub.S of the soil 2 in which the target 1 is buried can thus be calculated at step ST9.
Quite independently of such a process for measuring the dielectric constant .epsilon..sub.S, collection of the reflected wave profile data on a plane cutting through the soil at right angles with the ground is performed at step ST1. That is, the distance between the transmitting antenna 4 and the receiving antenna 5 is fixedly set to a predetermined value and both the antennas 4, 5 are moved in increments of a predetermined distance on the surface of the soil 2 in the direction at right angles with the direction in which both the antennas 4, 5 are disposed and transmission of a monocyclic pulse and receipt of its reflected wave are performed at every increment in the movement and thereby the reflected wave profile data on the predetermined plane cutting through the soil is obtained. In this reflected wave profile data, the reflected wave appears in the form of a hyperbola for each of the targets 1.
On the thus obtained reflected wave profile data, the monocyclic pulse propagated through the soil is distorted and largely attenuated depending on the distance it traveled, and further, it has a relatively high noise level, and therefore, the obtained reflected wave profile data is subjected to preprocessing such as filtering, level control, and the like for form shaping at step ST2.
Then, at step ST3, the thus preprocessed reflected wave profile data is subjected to synthetic aperture processing using the actual dielectric constant .epsilon..sub.S of the soil 2 calculated at the aforementioned step ST9 and thereby a certain image data is obtained. That is, as to each hyperbola on the aforementioned reflected wave profile data corresponding to each target 1, the data are made to cohere around the vertex portion, and thereby, targets sport according to weighting of their images are produced.
Since the thus obtained image data is expressed against the time scale, a geological correction is carried out at step ST4. That is, according to the fact that the propagating speed of the electromagnetic wave through the soil is inversely proportional to the square root of the dielectric constant of the soil, the scale of the aforesaid image data is converted from time scale to length scale using the actual dielectric constant .epsilon..sub.S of the soil 2 calculated at the aforesaid step ST9. The thus obtained image data expressed against the length scale is processed for outputting at step ST5 and output as a detected image output easy to observe.
Since the prior art underground object detecting system was constructed as described above, the collection of geological data required for calculation of the actual dielectric constant of the soil, in which the targets are buried, necessary for synthetic aperture processing, geological correction, etc. had to be performed as an operation completely independent of the collection of the reflected wave profile data for obtaining the detected imaage output, which has made complex the work of detecting the underground objects and this has been a problem with the prior art system.